: I have an old building that is occasionally filled with a sewer odor. The odor appears first in an area of the building that contains no fixtures except a couple of floor drains. I put water in the drains to make sure the traps were filled. The building has been remodelled so many times that anything is possible. Is there a visible "smoke" that can be introduced into the drains to help locate the leak?

Floor drains are the worst problem for office buildings. If I'm spending time upgrading plumbing, I fill the floor drains while I'm there. They should have trap primers on them to keep the seal on the traps. Terry

• Smoke testing